Arrays of detectors are used in imaging systems operating with high-energy photons, such as X-ray or y-ray cameras. Depending on its mode of operation, each detector of the array converts a detected photon, directly or indirectly, to a measurable electrical parameter, herein assumed to be a charge, which is a function of the energy of the detected photon. Typically, both the number of detected photons within a specific time period, and the energies of the detected photons, may be used to construct an image of the object generating the photons.
Once a detector has detected a photon, the detector is effectively disabled until the charge it generates has been read from the detector and the detector readout unit has been reset. Consequently, in the time period between charge generation and detector's readout unit reset, any photons incident on the detector are unread, so that in this time period the detector is unresponsive. Arrival of a photon at the detector during the unresponsive time period of the detector defines a first type of “simultaneous multiple-event.” For systems having even relatively low photon rates, the loss of information caused by this type of simultaneous multiple-event, leading to unread events, may critically affect the final image produced.
In some applications, the detectors may be set to only require readout for a specific range of levels of the charge, so that photons outside a corresponding range of energies are not detected. This process may relax, at least somewhat, the requirement for a fast detector readout, but may cause problems at the occurrence of a second type of simultaneous multiple-events.
The second type of simultaneous multiple-event is different detectors requiring readout during overlapping time periods. These type of events typically occur when the energy of one photon is effectively shared by a number of detectors. Such sharing may occur if the charge generated by absorption of the photon is distributed over more than one detector. It may also occur if the photon energy itself is spread over more than one detector, such as may occur with Compton scattering. Adjusting the detector to only respond to a specific range of levels may thus mean that photons causing the second type of multiple event go undetected.
Thus, a detector typically requires a fast readout.
U.S. Pat. No. 5,847,396 to Lingren et al., whose disclosure is incorporated herein by reference, describes a photon imaging system which may be used as a gamma-ray camera. The system includes detection modules, each of which has a plurality of detection elements. A “fall-through” circuit is coupled to the detection elements of each module. The fall-through circuit automatically finds only those elements having a valid event, and having found such an event, the circuit searches for the next element having a valid event.
U.S. Pat. No. 6,333,648 to Tumer, whose disclosure is incorporated herein by reference, describes a multi-unit readout chip for nuclear applications. The chip has a number of operational modes, including a sparse readout mode in which only units having signals greater than a threshold value are readout. The sparse readout mode is claimed to increase data throughput.
U.S. Pat. No. 6,917,041 to Doty et al., whose disclosure is incorporated herein by reference, describes an event-driven charge coupled device (CCD). Charges from pixels in the device are loaded into a charge delay register, and an amplifier detects if a pixel charge level of the loaded charges are above a threshold. The amplifier controls the register, so that the charge on a pixel above the threshold, and charges on neighboring pixels, are steered into a first-in first-out (FIFO) register for later measurement. Charges in the register that do not require measurement are discarded.
However, notwithstanding existing systems, an improved method for reading imaging detectors would be advantageous.